[antlr-interest] wildcard in tree grammar

Gavin Lambert antlr at mirality.co.nz
Sun Nov 30 23:15:04 PST 2008


At 09:29 1/12/2008, Terence Parr wrote:
 >not sure.  I guess i left out as it's weird.  When would you 
write:
 >
 >^(. ID).  It's always the root that says what kind of thing it 
is,
 >
 >right?

This is a completely fabricated example, but your tree structure 
might dictate that at that position you know it's either a 
^(ORDER1FN ID) or a ^(ORDER2FN ID) or a ^(ORDER3FN ID).  In one 
tree grammar you care about the distinction, while in another you 
don't.

Of course you could just write ^((ORDER1FN | ORDER2FN | ORDER3FN) 
ID), but if you know that there will never be any other 
possibilities then it's simpler to just write ^(. ID), especially 
as the number of variations increases.


And then of course there's Oliver's example, where he just wanted 
to traverse the (sub)tree regardless of structure (presumably to 
pretty-print it or something).

They might be unusual cases, admittedly, but they seem reasonable.



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